Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li6+xP1–xSixO5Cl

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1–xSixO5Cl (0.3 < x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10–6 S cm–1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1–xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites.


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: Comparison of XRD patterns and extracted sample purity for powder samples of Li7SiO5Cl when using (a) hydrated and (b) dehydrated Li4SiO4, Li2O and LiCl precursors.     8.5(4) 8.5(4) 8.5(4) 0.6(4) -0.6(4) 0.6(4) O2 8.6(5) 8.3(5) 9.2(5) 0.1(5) 0.6(4) 0.3(4) O3 8.0(4) 8.0(4) 8.0(4) -0. 12.7(4) 12.7(4) 12.7(4) -0.6(4) 0.6(4) 0.6(4) O2 13.5(5) 12.3(5) 12.7(5) -1.1(4) -1.6(4) -1.0(4) O3 11.9(4) 11.9(4) 11.9(4) -0.  Figure S3: Pawley fits against SXRD data at a)100 K, b) 300 K and c) 500 K for Li7SiO5Cl; Iobs (black circles), Icalc (red line), Iobs-Icalc (grey line) and Bragg reflections (black tick marks for Li6+xP1-xSixO5Cl, red tick marks for Li4SiO4, orange tick marks for Li6SiO4Cl2 and blue tick marks for LiCl; d) Ordering peaks disappearing as temperature is increased and Li7SiOCl transitions from P213 to 4 � 3 Figure S4: a) Lattice parameters as a function of temperature for Li7SiO5Cl b) DSC data collected for Li7SiO5Cl, the peak at 472.1(6) K corresponds to the phase transition from P213 to 4 � 3m in Li7SiO5Cl, the peak is broadened by the presence of the impurity phase of Li6SiO4Cl2 which exhibits a phase transition at a similar temperature. 1  and spectral deconvolution (dotted lines) of the ordered Li1 (T5a) and Li2 (T5) as well as the mixed occupancy Li3 (T3) and Li4 (T4) environments. The resonances associated with the largest Li4SiO4 impurity (~10 mol%) at 2, 1.2, and -0.7 ppm (red dotted lines) 2 as observed in XRD are also shown while the other impurities are below the NMR detection limit or unresolved due to the small shift range of 6 Li NMR. Figure S9: a) XRD patterns for Li6+xP1-xSixO5Cl. Black squares denote peaks corresponding to the cubic argyrodite phase and peaks corresponding to Si used as an internal standard are also present in some samples. b) Magnified views of the XRD patterns to highlight peak shift confirming the incorporation of Si and Li into the material. Figure S8: XRD patterns obtained when starting materials are a) hand ground and b) ball milled during the synthesis of Li6.3P0.7Si0.3O5Cl. Black squares denote peaks corresponding to the cubic argyrodite phase. Insets highlight the asymmetric peak shape in hand ground samples due to the presence of several distinct cubic argyrodite phases. When the starting materials are ball milled, a single argyrodite phase is formed as indicated by the symmetric peak shape. XRD patterns obtained when using c) hydrated and d) dehydrated Li4SiO4, Li3PO4, Li2O and LiCl starting materials.      The experimental spectra (full lines), total fit (dashed lines) and spectral deconvolution (dotted lines) of the different 31 P environments arising from differing second coordination spheres due to the site occupancies of neighbouring Li environments. The percentage contributions of each resonance to the overall line shape is available in Table S8. The vertical dashed lines highlight the decrease in chemical shift of the most intense resonance, arising from the increased statistical probability of a greater number of Li atoms in the 31 P second coordination sphere, due to the increasing Li content.   (Table S7). 20 Table S9: Summary of the extracted 31 P NMR parameters and the tentative assignments for Li6+xP1-xSixO5Cl (x = 0.3, 0.75 and 0.8) from the statistical probability according to the Li site occupancies observed in refinements against SXRD data (Table S7). x
These various NMR resonances arise as the 31 P nucleus is sensitive to the second coordination sphere, namely the T3, T4 and T5 Li sites. Hence when the site occupancy of these Li sites is less than 1, a statistical distribution of chemical shifts is observed. For example, the likelihood of a 31 P nucleus having 0 to 12 Li atoms in its second coordination environment will differ depending on the Li site occupancy of the varying sites. The second coordination sphere of 31 P in the three compositions analysed is complex as for all three phases there can be up to 12 second nearest neighbour Li atoms, 8 × T5 and 4 × T3/T4 Li atoms with varying site occupancies ( Figure 6).
The trend towards lower chemical shift with increasing x content can be explained through the higher concentration of Li atoms on the T3 and T4 position as this provides an increase in electron density in the second coordination sphere of the 31 P nucleus and hence an increase in the chemical shielding.
Moreover, the resonances were compared with the expected ratios of the different possible second nearest neighbour environments for phosphorous using the site occupancies of the different lithium sites obtained from structural refinement against SXRD data ( Figure S15) and the NMR resonances assigned from the relationship between coordination number and chemical shift (Table S8)

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The energy of activation obtained through 7 Li VT NMR was determined through the correlation of the onset temperature of motional narrowing Tonset and Ea: E a = 1.67 × 10 −3 • T onset (Eq S1) and the extracted Ea values are summarised in Table S10. The data in Figure S22 was fit to a Boltzmann sigmoid regression curve in order to determine the inflection point of the curve, Tinflection, where the Li ions have a jump rate, τ -1 of the order of rigid lattice regime, (ω/2π)rl, this expression takes the form of: where ω(T)/2π is the central transition linewidth at temperature T, (ω/2π)∞ is the residual linewidth in the fast motional regime and a is a fitting parameter.   Figure S24: a) Li6PS5Cl stability in air for which full decomposition occurs within 1 hour as indicated by the disappearance of cubic argyrodite peaks. b) Li6.7P0.3Si0.3O5Cl exhibits increased stability in air; decomposition starts after ~3h indicated by broadening of cubic argyrodite peaks and the appearance of Li2CO3 impurity peaks.